The global petroleum supply-demand contradiction is increasingly prominent at the beginning of the 21st century. Along with the increased demand for various petroleum and petrochemical products, the price of crude oil in the market is continuously increased. This situation leads to persisting high market-prices of important chemical products such as light olefins (especially ethylene and propylene) and light ethers (such as dimethyl ether, ethyl ether, etc), using petroleum and petrochemical products as feedstock. Therefore, it is one choice of solving these problems to seek for another substituting feedstock such as by-product ethanol from agriculture and forestry, and methanol prepared from natural gas or coal to produce light olefins and light ethers.
The process for preparing ethylene from ethanol is to carry out the dehydration reaction CH3CH2OH→CH2═CH2+H2O at 140-400° C. with a suitable catalyst. At the beginning of 1980's, India and Brazil built up industry-scale devices for converting ethanol to ethylene, using SiO2—Al2O3 as catalyst and adopting fixed bed and fluidized bed reactors altogether. For the balance of heat, an additional fuel will be added when the catalyst is regenerated.
The process disclosed in U.S. Pat. No. 6,441,261 is to convert oxygenates (methanol, etc) to light olefins, e.g. ethylene and propylene, on a silicoaluminophosphate molecular sieve catalyst under a relative high pressure.
U.S. Pat. No. 6,303,839 and U.S. Pat. No. 5,914,433 convert oxygenates (methanol, etc) to light olefins and fractionate out the propylene and/or butene therein for cracking, thereby enhancing the yield of ethylene and propylene. Although the above processes also use fluidized bed operation, it is seen from the data of the examples that the yield of coke is only 2%. With a low yield of coke, the heat of the system is difficult to be balanced, and an external heat supply is generally needed.
U.S. Pat. No. 6,049,017 increases the yield of light olefins by separating the product containing C4 components and converting them to ethylene and propylene on a non-molecular sieve catalyst. This process may be used in the catalytic cracking or the methanol dehydration for producing ethylene and propylene.
U.S. Pat. No. 4,148,835 uses a shape-selective molecular sieve catalyst and derivatives thereof to convert alcohols (especially methanol) to a product mainly containing light olefins, but this patent does not mention of the process.
The conventional process for preparing dimethyl ether (DME) from methanol is to carry out the methanol vapor phase dehydration with an acidic catalyst to give DME and other by-products such as CO, CO2, CH4, C2H4, H2 and the like. Said acidic catalyst includes zeolite, active alumina, crystalline aluminosilicate, silica/alumina, cation exchange resins and the like.
The alcohol dehydration reaction is conducted at a certain temperature. Although coke deposition may occur during the reaction, the amount of the coke is insufficient to balance the heat of the process. In summary, all the prior arts provide the heat in a manner of supplying an external fuel, making the process too complicated or the energy consumption too high.